The present invention relates generally to a manufacturing method, and more particularly to a manufacturing method of a multilayer mirror and an optical system in an exposure apparatus used to expose a plate, such as a single crystal substrate for a semiconductor wafer, and a glass substrate for a liquid crystal display (“LCD”). The present invention is suitable, for example, for an exposure apparatus that uses an ultraviolet (“UV”) light and an extreme ultraviolet (“EUV”) light as an exposure light source.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern of a mask or a reticle onto a wafer, etc. to transfer the circuit pattern, in manufacturing fine semiconductor devices using the photolithography technology.
The minimum critical dimension (“CD”) transferable by the projection exposure apparatus or resolution is proportionate to a wavelength of the light used for the exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Recent demands for finer processing to semiconductor devices have promoted use of a shorter wavelength of the UV light from an ultra-high pressure mercury lamp (an i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm).
However, the semiconductor devices become rapidly finer, and the lithography using the UV light has the limit. The projection exposure apparatus using the EUV light with a wavelength of 10 to 15 nm shorter than that of the UV light, which is referred to as an “EUV exposure apparatus”hereinafter, has been developed for efficient transfers of very fine circuit patterns smaller than 0.1 μm.
Since materials greatly absorb the light in a wavelength range of the EUV light, a dioptric optical element or system (that utilizes refractions of the light) for use with the visual light and the UV light is not viable. Therefore, the EUV exposure apparatus uses a catoptric optical element or system that utilizes reflections of the light. The catoptric optical element in the EUV exposure apparatus includes an oblique incidence total reflection mirror and a multilayer mirror.
The wave range of the EUV light has a refractive index's real part of slightly smaller than 1, and is totally reflected by increasing an incident angle to introduce the light almost parallel to the reflection surface. The oblique incidence total reflection mirror can acquire such a reflectance as 80% or greater with an oblique incidence with an angle between several degrees to 10° from the reflection surface. However, the oblique incidence total reflection mirror is not suitable for practical use because it makes an optical system large and has little optical design freedom.
The catoptric optical element for the EUV exposure apparatus uses a multilayer mirror that alternately forms or layers two kinds of materials having different optical constants on a glass substrate. The multilayer mirror can obtain a high reflectance at an incident angle close to the normal incidence. The multilayer mirror reflects an EUV light with a specific wavelength when receiving the EUV light. In other words, the multilayer mirror has a wave selection. For example, only the EUV light in a narrow bandwidth around a wavelength λ that is approximately expressed by Bragg's equation below is effectively reflected where θ is an incident angle, λ is the wavelength of the EUV light, d is a coating cycle, and m is an order:2×d×cos θ=m×λ  [EQUATION 1]
The multilayer film used for the multilayer mirror includes a W/C multilayer film that alternately layers tungsten (W) and carbon (C), a Mo/C multilayer film that alternately layers molybdenum (Mo) and C, etc. In particular, a Mo/Si multilayer film that alternately layers Mo and silicon (Si) has a high transmittance at a long wavelength side of the L absorption edge (wavelength of 12.4 nm) of Si, and can relatively easily obtain the reflectance of 60% or greater to the normal incidence near the wavelength of 13 nm. The multilayer mirror having the Mo/Si multilayer film is used for such R & D fields as an X-ray telescope and an X-ray laser resonator, and an application to the EUV exposure apparatus is expected.
The multilayer film is produced by the thin-film forming technology, such as a sputtering method, a vacuum evaporation method, and a CVD. For example, the Mo/Si multilayer film is produced by the sputtering method. However, the thin film produced by the sputtering generally has an internal compression stress. The internal stress generated in the Mo/Si multilayer film deforms the substrate in the multilayer mirror, and causes the wavefront aberration in the optical system. As a result, the optical characteristic of the multilayer mirror deteriorates.
Prior art propose various technologies to reduce the internal stress in the Mo/Si multilayer film. See, for example, Japanese Patent Applications, Publication Nos. 11-38192 and 2001-27700, and U.S. Pat. No. 6,011,646. More specifically, Japanese Patent Application, Publication No. 11-38192 proposes a method for controlling the internal stress in the multilayer film by adjusting the content of boron (B) in the Si layer. Japanese Patent Application, Publication No. 2001-27700 proposes a method for reducing the internal stress in the multilayer film by forming a ruthenium (Ru) layer in at least one of the Mo layers. U.S. Pat. No. 6,011,646 proposes a method for cancel the internal stress in the multilayer film by forming, between the substrate and the multilayer film, a stress compensating layer that has a stress in a direction different from or opposite to the internal stress direction in the multilayer film.
The methods for including B or for forming the Ru layer in the thin-film layer in the multilayer film reduce the internal stress by modifying the coating structure that maximizes the reflectance of the multilayer film. These methods effectively reduce the internal stress in the multilayer film, but inevitably cause the deterioration of the reflectance. In addition, since these methods need to add a additional layer (made of an additional material) to the Mo layer and the Si layer, the film forming apparatus and procedure become complex, causing problems of a long film forming process and a deteriorated film forming precision.
On the other hand, the method for canceling the internal stress using the stress compensating layer causes a minute roughness (smaller than 1 nm) on the stress compensating layer, and lowers the reflectance of the multilayer film on the stress compensating layer. The multilayer mirror preferably dispenses with the minute thickness distribution or film thickness unevenness in the mirror plane. However, the minute roughness on the surface of the relaxation stress layer appears on the multilayer film, and the reflection surface shape precision of the multilayer film lowers (or the reflection surface shape cannot be turned into a desired shape). The optical system that includes the multilayer mirror causes a large wavefront aberration.